The use of fiber-reinforced composite materials consisting of reinforcing fibers and matrix resins has been widely extended to the fields including aerospace, sports, and general industry fields, because fiber-reinforced composite materials make it possible to design materials that have benefits of both reinforcing fibers and matrix resins.
As reinforcing fibers, glass fibers, aramid fibers, carbon fibers, boron fibers, and the like may be used. As matrix resins, both thermosetting resins and thermoplastic resins may be used, but thermosetting resins are more frequently used because reinforcing fibers can be more easily impregnated the thermosetting resins. As thermosetting resins, epoxy resins, unsaturated polyester resins, vinyl ester resins, phenolic resins, maleimide resins, cyanate resins, and the like may be used.
For producing fiber-reinforced composite materials, various methods such as prepreg method, hand lay-up method, filament winding method, pultrusion method, RTM (Resin Transfer Molding) method, and the like may be used.
Among them, the RTM method where a reinforcing fiber substrate placed in a mold is impregnated with a liquid thermosetting resin, and heated to cure has a great advantage that a fiber reinforced composite materials of complicated shape can be molded.
Recently, there has been a need for producing fiber-reinforced composite materials of high fiber volume fraction (Vf) (particularly more than about 45%), which are lightweight, and excellent in mechanical properties such as strength and elastic modulus, by using the RTM method. However, it has been difficult to efficiently produce fiber-reinforced composite materials with high Vf in a short time period using the conventional RTM method.
In the RTM method, the packing fraction of reinforcing fibers in a mold should be high to produce fiber-reinforced composite materials with high Vf, because the Vf of a product is mainly determined by the packing fraction of reinforcing fibers in a mold. If the packing fraction is high, permeability is low, because high packing fraction means low void fraction. And if the permeability is low, injection time of the resin composition is lengthened.
If the thermosetting resin composition is heated to cure at a constant temperature, viscosity of the liquid composition increases, and then, gelation occurs. After gelation, rubbery polymer is obtained. The glass transition temperature of the polymer increases as the curing reaction progresses. If the glass transition temperature exceeds the curing temperature, the polymer turns to a glassy polymer. In general demolding is carried out after vitrification. For general thermosetting resin compositions, the ratio of the time required from the beginning of injection to vitrification to the time from the beginning of the injection to a point during which the thermosetting resin compositions maintain liquid phase with a viscosity adequate for injection is usually greater than 6.
In cases of producing fiber-reinforced composite materials whose Vf is not high, it is possible to carry out the method in a short time (several minutes or about ten minutes), where injection is terminated before the viscosity of the thermosetting resin compositions becomes too high, and curing for a predetermined time and demolding are carried out while maintaining the mold temperature constant, because injection time of the resin composition can be short. This method is often called S-RIM (Structural Reactive Injection Molding).
However, in cases fiber-reinforced composite materials with high Vf, it is impossible to carry out the same method mentioned above at the mold temperature at which the curing reaction is terminated in a short time, because rapid increase of viscosity, and furthermore, gelation occurs during impregnation. On the other hand, if the temperature or the reactivity of the thermosetting resin composition is lowered to prevent rapid increase of viscosity during impregnation, the time before the demolding is increased, and the overall molding process time is increased. To decrease molding process time, raising the temperature of the mold is often used after the termination of injection. This method, however, is not sufficient to decrease total molding process time because the method requires additional time to raise and lower the temperature of the mold.
The object of the present invention is to provide the epoxy resin compositions that have a low ratio of time required from the beginning of the injection to vitrification to time required from the beginning of the injection to a point during which the thermosetting resin compositions maintain liquid phase having viscosity adequate for injection.
Epoxy resin compositions that are similar to those of the present invention are disclosed in Japanese patent laid-open publication No. 1978-113000. These epoxy resin compositions comprise epoxy resins, imidazole derivatives, methanol and/or ethanol. In these epoxy resin compositions, methanol and/or ethanol function as solvent, and occupy a large proportion of the compositions. The above patent document also states that methanol and (or) ethanol is volatilized before curing. If such epoxy resin compositions are injected into a mold and heated to cure, it is impossible to volatilize the methanol and/or ethanol. If curing is carried out in the presence of large amounts of methanol and (or) ethanol, cured resin products with crosslinking structure cannot be obtained or cured resin products having very low crosslinking density is obtained. Therefore, these epoxy resin compositions have not been used as a matrix resin for the RTM method.
Japanese patent laid-open publication No. 1990-103224 discloses epoxy resin compositions comprising epoxy resins, an imidazole derivative, boric acid and mannitol. In the compositions disclosed by the above patent, solid bodies prepared by grinding the mixture of imidazole derivatives, boric acid and mannitol is used to blend with the epoxy resins. However, if the reinforcing fiber substrate is impregnated with such epoxy resin compositions, heterogeneity of the composition is raised because the solid bodies hardly penetrate into bundles of reinforcing fibers. Thus, curing of the resin compositions is insufficient in some regions, and cured resin products' having high glass transition temperature cannot be obtained. Therefore, these epoxy resin compositions have not used as a matrix resin for the RTM method.
Journal of Applied Polymer Science, Vol. 30, pp. 531-536 discloses a mixture comprising p-cresol glycidyl ether, an imidazole derivative, and isopropyl alcohol. However, if this mixture is reacted, the resulting product is a soft linear polymer without a crosslinking structure. Therefore, this mixture cannot satisfy requirements of high glass transition temperature and strength necessary for a matrix resin in a fiber-reinforced composite material.
None of the above compositions or mixtures can increase glass transition temperature even when heated to induce reaction, and are not suitable for advantages such as provided by the present invention which are to decrease the ratio of time from the beginning of the injection to vitrification, to the time from the beginning of the injection to a point during which the thermosetting resin compositions maintain liquid phase having viscosity adequate for injection.